Students visit Pillow basalts and breccias in Albay, Philippines!
The 2024 International Earth Science Olympiad (IESO), the first to be conducted in face-to-face mode since the COVID-19 pandemic winded down, was held in Beijing, China from August 7 to August 17. Among those who participated there were students from the Philippines. This article concerns some of the geological sights and sounds of Albay province that were visited by the finalists of the Philippine Earth Science Olympiad (PESO).
Albay, which lies southeast of the Philippine capital, Manila, is famous among tourists and geologists alike for its Mayon Volcano, whose near-perfect cone shape sets it apart from other volcanoes in the world. It is also among the Philippines’ proposed UNESCO World Heritage sites. The Mayon is also the Philippines’ most active volcano, with its most recent magmatic eruption just in June 2023. However, the province has interesting geological features and stories that are not as well-known as that of the Mayon Volcano’s geological story.
To recall, the Philippines held two rounds of selections to determine who will represent the country in the world’s sole competition involving the field of geosciences and terrestrial astronomy for high school students. The first round, which was held simultaneously in four cities in the country last April helped determined the top 10 finalists, who were then interviewed virtually by a panel of Filipino Earth and Space scientists. The top 4 students who would represent the Philippines were selected based on their assessments.
Although only four students will be part of the Philippine delegation to the 2024 IESO, all ten finalists and their teachers were invited to the awarding ceremony and a geology tour was organized by the Philippine Earth Science Olympiad organizers last June 20-21. I also had the privilege to join the students and the teachers in the tour held in Albay province (Figure 2).
Stop 1. Mirisbiris Pillow Basalts, Sto. Domingo, Albay
The venue of the awarding ceremony, the Mirisbiris Nature and Garden Center in the town of Santo Domingo, was itself geologically appropriate and relevant. It is owned by Glenda Newhall and her spouse, former USGS volcanologist Christopher Newhall. Below the couple’s property lies a beachside outcrop of pillow basalts far older than that of the Mayon volcano’s age (Figure 3).
The outcrop of pillow basalts (also called pillow lavas) is thought to belong to the Rapu-Rapu Ophiolite Complex (ROC). Ophiolites like the ROC are a piece of oceanic lithosphere that got placed onto land some time after they have been displaced away from a divergent plate boundary, such as a mid-oceanic ridge, in the course of seafloor spreading. Ophiolites have also been used as evidence for the plate tectonic theory since they serve as remnants of old oceanic lithosphere that would have otherwise been consumed during subduction. Their existence amid the dearth of oceanic lithosphere older than 200 million years ago can only be explained by the mechanisms of colliding and subducting plates.
Let us review our science class for a bit. If we recall, the plate tectonic theory states that the outermost rigid part of the Earth, called the lithosphere, is broken up into large slabs of rock called plates. There are two kinds of plates: oceanic, which is denser, and continental, which is more buoyant. Suppose you have two oceanic plates colliding with each other (Figure 4). Then imagine slicing these two plates from the side. One of the two plates is ultimately lighter than the other. The denser of the two sinks under the lighter one. However, in the process of collision, parts of the lighter oceanic plate could become crushed and destroyed. This destructive process may also leave behind remnants of the lighter plate, which could eventually end up on land as ophiolites. The Philippines contains belts of these ophiolites and in many areas, serve as the “basement rock” or the oldest rock unit.
In an idealized ophiolite sequence (Figure 5), the base is formed by peridotites, representing the upper mantle, which are in turn overlain by gabbros, sheeted dikes, pillow basalts, all of which represent the crust. In particular, the pillow basalts come from lava erupted from the oceanic ridge and hardened upon exposure to seawater. The runny lava, as well as the pressure exerted by the overlying seawater helped sculpt the basalts into pillow-like shapes, hence the term. The basalts are finally capped by a “pelagic cover” of sedimentary rocks such as siltstones, cherts, or sandstones, representing the sediments that settled on the deep ancient seabed.
In reality, because of weathering and erosion as well as tectonic movement over millions of years, only parts of the sequence tend to remain exposed for geologists to study. Intact, well-preserved sequences are rare especially in a tropical and geologically active country like the Philippines.
In the Mirisbiris property, only the pillow basalt component of the ROC is exposed. The peridotites associated with the ROC lie farther in the islands of Rapu-Rapu, to the east of where Mirisbiris is, but we were unable to visit them. Nevertheless, the pillow shape of the basalts are quite distinct with the pinch structures being especially prominent. Pinch structures are the teardrop-like shape that pillow basalts have when cut and viewed from a side. The tip of a pillow basalt’s pinch structure points to where the bottom is while its convex side points where “up” is. Thus, the orientation of these structures can help determine whether a place has been tilted significantly by tectonic forces or not. As can be seen in Figure 6, the pinch structure appears to have been tilted towards the right from an initially upright position.
The age of the ROC itself has been constrained to be no younger than 79 million years old or late Cretaceous (Peña, 2008), although the pillow basalts themselves have not yet been radiometrically dated. Their presence in the area, as well as the likely age of the ROC, indicates that the beachside property, was under the sea at a time when dinosaurs were roaming other places. Most, if not all, of the basement rocks comprise the present-day Philippines was submerged under the sea during the age of the dinosaurs so there is very little hope that dinosaur fossils would be discovered in the country. The Mayon Volcano, along with its sister volcanoes along the Bicol Volcanic Arc did not yet exist at that time, only to emerge relatively recently, at around 20,000 years ago (Van Westen et al., 1994).
Stop 2. Calayucay breccia, Sto. Domingo, Albay
Around 6 kilometers northeast of the property, in the village of Calayucay, there is a beach-side outcrop of breccia (Figure 7) that may form part of the pelagic cover of the ROC. Breccia is a type of clastic sedimentary rock, which means it came from the deposition and diagenesis of weathered pre-existing rock. The angular to sub-angular shape of the clasts suggests that the source material of the breccia did not come far from where it settled and underwent diagenesis. Yet the size of the clasts is somewhat uniform, with only occasional clasts being larger than 3 cm. Most clasts where between 1 to 2 cm (granule-sized) in diameter. The lithology of the clasts was mostly andesite or sandstone and notably no limestone. If the breccias are indeed part of the ROC’s deep marine pelagic cover, the uniform size and lithology as well as the absence of shallow marine fossils and sedimentary rocks like limestone may lend credence to this conjecture.
We’ve also had other stops such as our attempted search for outcrops of Mayon’s cataclysmic 1814 eruption and the Hoyop-hoyopan cave. These places have their different stories from that of the pillow basalts and the breccias. However, what transpired during our trips there would be revealed in another article.
References:
Peña, R.E. (2008). Lexicon of Philippine Stratigraphy. Mines and Geosciences Bureau.
Van Westen, C.J., Dayao, A., and Voskuil, R. (1994). “Geomorphology of the Mayon Volcano and its relation to hazards.” Unpublished MGB Report.
Stern. R.J. (2002). Subduction zones: Reviews of Geophys-ics, v. 40, doi: 10.1029/2001RG000108.
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